Development of offshore oil fields requires pipes commonly referred to as risers or flowlines. A riser allows a wellhead located at the sea bottom to be connected to a platform located at the surface. A flowline is an underwater pipe lying on the sea bottom and connecting two wellheads for example. The pipes can be flexible or rigid.
During pipe installation, a floating vessel brings to the oil field site the pipe that can be wound on reels or stored in sections whose length ranges between about 20 and 30 meters. The pipe is progressively arranged rectilinear until the lower end reaches the subsea wellhead. Unwinding of the pipe can be more or less fast depending on whether flexible pipes, unwound rigid pipes, or welded, flanged or screwed rigid sections are used.
According to standard practice, the end of the pipe is filmed as it gets close to the wellhead. An operator can thus know the relative position of the end of the pipe in relation to the wellhead. Positioning of the lower end of the pipe is conditioned by two motion types of the floating vessel. On the one hand, the vertical and horizontal motion, at the frequency of the wave motion, considered as an elliptical motion if the wave motion is assumed to be sinusoidal. On the other hand, a low-frequency drift motion, of the order of some minutes. When the end of the pipe is vertical to the wellhead, the operator orders to apply a descending vertical motion to the pipe so that the end of the pipe comes smoothly into contact with the wellhead. Then, the end of the pipe is connected to the wellhead.
The wellhead connection operations are random, delicate and depend on the operating staff's skill. The floating vessel undergoing the wave motion and the low-frequency drift, the motions of the lower end of the pipe are not controlled. It is necessary to wait for the appropriate time where the end of the pipe is vertical to the wellhead to be able to act rapidly and to establish the connection.
Furthermore, this method cannot be implemented in bad weather. Under the effect of the wave motion and of the wind, the floating vessel undergoes motions, notably vertical motions, of great amplitude transmitted to the lower end of the pipe. These motions make connection to the wellhead difficult or impossible.